Page:Concepts for detection of extraterrestrial life.djvu/41

 CHAPTER VIII

To determine the presence of life on other planets through remotely controlled instruments, two factors must be considered. First, we postulate that life on other planets has a chemistry similar to ours. This is an obvious first guess and may well prove to be approximately correct. As was shown in the introduction, there is reason to believe that the chemical events occurring on the primitive Earth also occurred on other planets of the solar system. Secondly, what is life and how do you know when you have found it?

“This,” says Dr. Ira Blei, “brings us to the heart of the problem of the search for life on other planets.” Dr. Blei, of Melpar, is the scientist in charge of the optical rotatory experiment for NASA.

How can one design single experiments which will provide enough information to permit a decision to be made concerning the existence of biologically significant molecules? Life has become very difficult to define in just a few words. The highest form of life on Earth, man, is a collection of very complex molecules having certain life-like properties associated with them. At the other end of the scale are the many types of simple chemical substances—sugars for example—which are obviously not alive. Further, somewhere between the two extremes are systems which are sometimes “alive” and sometimes not: the viruses.

So, we may look for a substance or property which is common to all life. One measurable property which has consistently been found in all living systems is optical rotation. A substance is said to possess optical activity when a “flat ribbon,” or plane wave, of light (polarized) passing through this substance is twisted, or rotated, so that the flat ribbon of light emerges in a new plane.

This ability to rotate the plane of polarized light is associated with molecular structure in a unique manner, just as the absorption spectroscopic characteristics are unique. Not all materials are capable of rotating the plane of polarized 31